Re: {Chrysler 300} coolant level down

Just to add my 2 cents to this... I have a '56 Windsor with a 331 poly 2 bbl, which I have owned for 48 yrs. As most of you know a good portion of the engine is set up like a full hemi. The manual calls for a 7 psi cap and I never had an issue with the radiator. .... John

In a message dated 5/13/2025 12:35:14 AM Eastern Daylight Time, henry.schleimer@xxxxxxxxxxxxxxx writes:

raises a point about the pressure that a cap needs to hold in radiators designed to have an expansion tank on top rather than an overflow. This has always been a controversial topic but I’ll add my two cents worth in support.

When I got my 300C radiator re-cored I went to an expert with a lifetime of experience and who actually makes cores for vintage cars. Talking to him about my parents’ 67 Valiant with a similar radiator design, I bemoaned the fact the top of the tank split along the solder line three times over its 27 year life. He explained his thoughts. Older radiator tanks were wider than the core (to give it volume) but that meant the force trying to rip the seam (pressure x area) was about twice the modern narrower design putting greater stress on the solder. To make sure I never had this problem, he said to use a 7 psi cap, not the factory 14 psi. He said I won’t have a problem of losing coolant and I’ll never have to re-solder the top if the rest of the system is ok. As proof he said it was always the top tank that lifted, never the bottom, because the bottom tank was narrower (and less force on the solder).

I’m prepared to give it a go. Nothing to lose by trying and much to gain in reducing the stress on the tank, hoses etc. If I lose too much coolant too soon, I can always just increase the psi rating or just top it up more often.

I just recalled that my father just used plain water in the radiator back in the day, fifty years ago. So, I just looked up a chart and it shows that plain water will boil at 252 F under 16 psi pressure, but 60% coolant mix will boil at a similar 253 F under 8 psi. So, you can halve the radiator cap pressure for the same outcome (boiling) just by using coolant. Maybe the old “large” radiators were sized for the owners who didn’t bother with coolant? I wouldn’t try reducing the cap pressure on a modern car like John says as they don’t have a lot of coolant to spare increasing the chance of temperature spikes.

Cheers

Henry